Is a seastar able to drive the extinction of coral reefs?
It is the subject of most of the scientific studies and recent documentaries. It is attracting attention and funding from the most important Universities and research institutions around the Indo-Pacific. Questions are still opened about the corallivorous Crown-of-Thorns seastars (CoTS). The seastar belongs to the genus Acanthaster and there are at least 5 species, although studies are revealing the possibility of more. Acanthaster means seastar covered by thorns (ákantha, “thorn” + astḗr, “seastar”) which is an appropriate name. Looking at the seastar, we will find an alien-like body with up to 21-24 arms, the dorsal or aboral side is covered by sharp thorns, each with a toxin for defence purpose. The ventral or oral side is composed by thousands of tube feet which are used for walking and crawling the rough surface of the seabed, while a kraken-like mouth is located in the middle.
They are commonly found on coral reefs in the Indo-Pacific Ocean. Here, they can find their favourite food: corals. The main reason why the scientific community is focusing the attention on this seastar is the ability of population outbreaks or demographic explosions. During the outbreak, the population may increase the number of individuals of more than 100-fold, going from 3 or 4 individual up to 1000 per hectare. This means that they could potentially eat an entire reef in a short period of time. They have been considered as one of the worst plague of recent time for the coral reefs, consuming up to 80% of corals during an outbreak. Recently, control efforts have been developed using injections of vinegar or hand-removal from reefs to decrease the outbreak effect on corals. However, it seems that the effort is not effective because of the insufficient knowledge of the biology and ecology of the seastar. Thanks to future studies we will understand the causes and prevent rather than applied a strategy when is too late.
In the Maldives, outbreaks have been reported in 1989-90 by researchers and 2014-16 by the team of marine biologists of Athuruga and Thudufushi Resorts. The papers, published on scientific journals, reported the presence of more than 1000 CoTS on reefs around Athuruga and Thudufushi. These outbreaks have been one of the cause of the decrease in coral cover. Our resorts highlighted the importance of the scientific research to better understand the conditions of the Maldivian coral reefs and drive conservation efforts.
Thus, which are the causes behind a CoTS’s outbreak?
Despite more than 30 years of research on CoTS, there is no answer to this question. Indeed, the determinants and mechanisms regulating outbreaks remain elusive. However, several hypotheses aim to identify the main determinants in terms of both natural and anthropogenic sources such as nutrient enrichment, overfishing and the global rising of temperature.
Those factors may increase the survival of CoTS’ larvae and reduce the predator pressure on larval and adult stages causing inevitably an unbalances in the ecosystem.
Although CoTS may damage the coral reef, their presence may favourite the variability of corals with a positive effect on the coral diversity. Moreover, sporadic outbreaks may not necessary cause the local extinction of corals but lowered the dominant coral species in favour of the less abundant in order to increase the diversity of species along the reef. The high diversity of coral species is one of the proof of the good health of a coral reef.
The real problem is related to the increasing of the frequency of outbreaks, so the local extinction of corals may be indirectly driven by the human activities.
Surely, further studies will unlock the CoTS secrets but we are afraid that these secrets will definitely throw light on the responsibility of human activities.
For further information:
Article of National Geographic: